N.B. An asterisk before a word means it has its own entry in the glossary.

'hdmx' table

This is an optional TrueType table containing precomputed values of *LSB and *advance width for each *glyph at a variety of common *ppem values. Text layout in applications can thereby be faster. (Without precomputation, each glyph must be hinted even if the bitmap were not required.) See Apple's spec or Microsoft's spec for the format in detail.

'head' table

The mandatory TrueType table where certain global information is stored, including the *unitsPerEm value. See Apple's spec or Microsoft's spec for the format in detail.

In general, a "hint" is an instruction within the data of a particular *glyph, requesting the *rasterizer to invoke special handling for a particular region of its *outline. Hinting is one approach to the *aliasing problem in rendering scalable graphics on low-resolution devices. In practice, only typographic systems seem to use hinting.
In TrueType, hints are programmed by *instructions in TrueType's low-level hinting language. See the TrueType Hinting page.

'hmtx' table

This is a mandatory TrueType table, containing the default *LSB and *advance width for every *glyph in the font. These values are scaled and (if the font designer decides) hinted in the rasterizer. See Apple's spec or Microsoft's spec for the format in detail.